One of the most common assumptions people make is that child support automatically stops at a child's eighteenth birthday. There are a number of reasons, however, that this might not be what happens in your case.
In California, one major reason that support might not end when your child turns 18 is that he or she has a mental or physical disability that leaves him or her "incapacitated from earning a living" and "without sufficient means." In that case, each parent is expected to continue paying support "to the extent of their ability" indefinitely.
It sounds fairly simple, but a case like this can actually end up being very complicated because of the possibilities involved and the wording of the law.
For example, two parents may differ on whether or not the child is genuinely incapable of earning a living or merely disinclined to try. If there's some disagreement, then the young adult needs an independent medical exam to confirm the mental or physical disability.
There's also the question of how "without sufficient means" is defined. A young adult with a disability can be determined by family court to be incapable of working but rejected for Social Security benefits because of that program's strict requirements. That would likely leave the child destitute, then, without parental support.
However, if the young adult child files for Supplemental Security Income through the Social Security Administration and is approved, that would at least provide him or her some sort of income.
At that point, the court will probably look at the extent of each parent's ability to provide additional financial security for the adult child. If you and the other parent aren't particularly well-off, the Supplemental Security Income may be enough to satisfy the court and reduce or eliminate the support you have to pay.
On the other hand, if you are of considerable financial means, the court may impose additional significant financial liability on you and require you to support your child indefinitely. At that point, it might become necessary to make sure that the support operates through a trust, so that your child continues to maintain eligibility to medical services that he or she gets through the SSI program.
Child support for a disabled adult child can be a contentious and difficult issue to settle, but an attorney early in the process can be very beneficial.
Source: California Legislative Information, "Family Code - FAM Division 9. Support [3500-5700.905] Pat 2 Child support [300-4253] Chapter 1. Duty of Parent to Support Child [3800-3952] Article 2. Support of Adult Child [3910-3910.]," accessed Feb. 19, 2017